Ejector device

Abstract: Vacuum generating means for providing reduced pressure for sewage transport in a vacuum sewer system, which means comprises a liquid-driven ejector (3), the working medium of which is fed to the ejector (3) by a circulation pump (6) from a sewage collecting container (5), the suction side (4) of the ejector 3 being, via a check valve (9), connected to a vacuum sewer network (2) . Sewage delivered through the sewer network flows through the ejector (3) into the collecting container (5). The bore of the discharge pipe (11) of the ejector, is substantially cylindrical throughout. Its length (L) is 8 to 20, preferably 10 to 15, times the diameter (D) of its bore and the pipe (11) discharges directly into the open interior of the collecting container (5).

Description: The invention relates to vacuum generating means for a vacuum sewer system and in particular to the use of an ejector as a pump in such a sewer system. Ejectors have long been used as a source of partial vacuum in vacuum sewer systems. Such an arrangement is shown in US-A-4034421. According to this known art, the working medium of the ejector is a flow of liquid fed by a circulation pump to the ejector from a sewage collecting container. The suction side of the ejector is connected to a vacuum sewer via a check valve so that air and sewage delivered to the sewer are drawn through the ejector to the collecting container. The total efficiency rate of such a vacuum generating means is only about 5%. This is because the efficiency rate of the circulation pump is about 40% and only about 10% to 15% of its useful power can be utilised in the ejector powered by it. An improvement of the efficiency rate of the vacuum generating means is, however, not usually of any great importance per se. Improving the total efficiency rate of a liquid-driven ejector working as an air pump has been subject to extensive research. Nevertheless, the aim of the present invention is not to improve the efficiency rate of the ejector. This invention is based on the idea that in the special application of an ejector as the vacuum generating means for a vacuum sewer system it is more important to maximise the air flow drawn into the ejector at a sufficiently high vacuum level (higher vacuum = smaller absolute pressure). In a vacuum generating means according to the invention, the ejector discharges directly into a container (e.g. at atmospheric pressure). Under these circumstances the pressure and the kinetic energy of the mass flow from the ejector is not utilised at all and this has a decisive influence on the optimising of the function of the ejector. One aim of the invention is to optimise the function of a vacuum generating means using an ejector according to the preamble of claim 1 in an operational environment typical for vacuum sewer systems. It is important that a sufficiently high vacuum is generated by the ejector and that at the same time the air flow volume through the ejector is maximised. A typical vacuum level in a vacuum sewer system is about one half of atmospheric pressure, but considerable variations from this vacuum level occur in different applications. What constitutes the invention is set out in the following claim 1. The diffuser traditionally used in an ejector (that provides a conically enlarging end portion of the discharge pipe in the flow direction) is not required so that in a vacuum generating means in accordance with the invention, the bore of the discharge pipe of the ejector is substantially cylindrical over a length L which is 8 to 20, preferably 10 to 15, times the diameter of the bore. It is also important that the ejector, as known per se, discharges directly into the open interior of a collecting container, that is, not into a pipe connected to the collecting container since such a pipe could be narrow enough to affect the functioning of the ejector. It has been found that vacuum generating means utilising an ejector constructed in the manner described operates considerably better in the operational environment of a vacuum sewer system than corresponding traditional ejector-based vacuum generating means. It has also been found that, in some applications, two ejectors in a means according to the invention provide the same function as five traditional ejector-based vacuum generating means. This is in spite of the fact that the theoretical efficiency rate of the ejector used in a vacuum generating means according to the invention is possibly inferior to the efficiency rate of known ejectors. When applying the invention, it is of advantage that the pressure generated by the circulation pump, just upstream of the ejector, is at least 1.5 bar gauge, preferably at least 1.9 bar gauge. The use of such a high supply pressure enhances the air pumping capacity of the ejector and the flow rate of the working medium through the ejector. Thus it is recommended that the rate of flow from the circulation pump to the ejector is at least 90 m 3 /h, preferably about 100 m 3 /h or more. In order to obtain a sufficiently high pumping capacity in the ejector, it is of advantage that the cross-sectional area of the bore of the discharge pipe of the ejector is at least 2.2 times, preferably at least 2.5 times, the area of the smallest aperture in the nozzle of the ejector through which the working medium flows to generate the required vacuum in the suction chamber of the ejector. These values are advantageous especially in combination with the above-mentioned values for the pressure and the flow rate of the working medium used in the ejector. The said ratio should not be so high that a sufficiently high vacuum cannot be generated by the ejector. A suitable maximum value is usually about 3 to 3.5. The pumping capacity of the ejector is also advantageously affected by using an inclined connection of the end portion of the vacuum sewer to the suction chamber of the ejector. The angle of the sewer pipe relative to the longitudinal axis of the ejector is desirably 45° ± 20°, preferably 45° ± 10°. In conventional ejectors the angle in question is about 90°, but a connection inclined so as to reduce the change in the angle of flow of material drawn through the ejector has been found to be considerably more advantageous. Because a vacuum generating means according to the invention may have to operate in different vacuum sewer systems under different operational circumstances, it is desirable that the characteristics of the ejector can be adjusted so that the ejector in any application operates at or close to its optimum performance. The desired vacuum level, the amounts of air and sewage to be pumped may in different applications vary considerably. For cost reasons the circulation pump is desirably relatively small. The pump should be selected so that it can provide the desired ejector capacity. The ejector has to be adapted to the flow rate and the pressure generated by the selected circulation pump. Because the characteristics of an ejector cannot be adjusted by means of a simple adjustment device, the ejector is preferably so devised that its nozzle and discharge parts, are removably attachable to other structures of the ejector, so that by exchanging them for other parts, the characteristics of the ejector can be modified as required. The circulation pump may be used for two purposes. Primarily the circulation pump works as the ejector's energy source, but the sewage collecting container must also be emptied from time to time. If the power of the circulation pump is high enough, the emptying may be accomplished even when the circulation pump is simultaneously powering the ejector. If the power of the circulation pump is too low, the ejector must be shut down during the emptying phase of the collecting container by shutting off the working medium flow from the pump to the ejector. In a preferred embodiment, no such shutting off means is needed, instead the circulation pump is so powerful that it, even when the ejector is in operation, is capable of pumping a part of the circulating liquid to a height that is at least 10 metres, and preferably at least 15 metres, above the level of the pump. This makes it possible, when applying the invention to the vacuum sewage arrangement of a ship, to empty the collecting container without interrupting the function of the ejector while the ship is in harbour. The ejector would not normally operate continuously. Its function will be dependent on the vacuum level existing in the vacuum sewer network. The pressure rises in the sewer network whenever a W.C. toilet bowl or other device connected thereto is emptied. When the pressure in the network rises above a certain limiting level, the ejector can be started automatically and can then run until an adequate vacuum level is again attained in the sewer network. The collecting container is continuously maintained at atmospheric pressure. The diffuser conventional in the discharge pipe of prior art ejectors should not be used in the ejector of a vacuum generating means according to this invention. This is because the mass flow from the ejector is freely discharged into the interior of a collecting container. If, in the discharge area of the ejector there is an adjacent obstacle, for example a wall of the collecting container, this may have an unfavourable influence on the functioning of the ejector, especially if the distance between the outlet end of the discharge pipe and the obstacle is small. Therefore, it is recommended that the clearance between the end of the discharge pipe and the closest obstacle in front thereof is at least 0.5 metres, preferably at least 1.5 metres. It is important also to keep the discharge area of the ejector free from disturbing constructions in lateral directions (e.g. in a radial direction with respect to the discharge pipe). With regard to lateral clearances, the minimum desirable free area is considerably smaller, usually only 1.5 times, preferably twice the diameter of the discharge pipe measured from its longitudinal axis. A problem can be created by solid, semi-solid and fibrous matter and also rubber matter (such as condoms) present in normal sewage. For disintegrating these materials it is known to use grinding devices, integrated in the system. It has, however, been found that the use of a grinding device in a vacuum sewer network slows down, in a disadvantageous manner, the passage of sewage to the collecting container. Therefore, in a vacuum generating means according to the invention, a grinding device is not used in the sewer network itself, but instead is located in the circulation path of the circulation pump, preferably just upstream of the circulation pump. A grinding device in this location does not disturb the flows in the sewer pipe network and at the same time it considerably improves the operational conditions of the ejector and the homogeneity of its working medium. By this means the grinding device has a favourable effect on the working capacity of the entire vacuum generating means. The grinding device may, as known per se, be integrated with the circulation pump so that the drive motor of the circulation pump directly drives both the grinding device and the pump. Because there is normally no grinding device in the sewer pipe of a vacuum generating means according to the invention, solid and semi-solid matter in the sewage may, especially when the ratio of liquid to solid matter is low, cause clogging of the ejector. Normally, this happens very rarely, but for improved security it is recommended that in the housing of the ejector, or at the point where the vacuum sewer network joins the ejector, there is an inspection opening with an openable cover, through which any matter disturbing the function of the ejector may be removed when necessary. The invention will now be described more fully, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically shows the general arrangement of a vacuum sewer system employing a vacuum generating means according to the invention, and Figure 2 schematically shows a longitudinal section through the ejector of the system of Figure 1. In Figure 1, numeral 1 indicates W.C. toilet bowls connected to a vacuum sewer network 2. In the sewer network a partial vacuum of about 50% of atmospheric pressure is generated by an ejector 3. The number of toilet bowls 1 may be up to one hundred or more per one ejector 3. In close vicinity to each toilet bowl 1 there is a normally-closed sewer valve la that directly joins the interior of that toilet bowl 1 to a sewer pipe held under the partial vacuum. A suction pipe 4 of the ejector 3 is connected to the outlet end 2a of the sewer network 2. The ejector 3 discharges into a sewage collecting container 5. A powerful circulation pump 6 draws the mainly liquid sewage present in the container 5 from the container through a pipe 7 and pumps it through a pipe 8 to the ejector 3, where the flow delivered by the pump 6 acts as the working medium for operating the ejector, so that a partial vacuum is first generated in the suction chamber (which includes the pipe 4) of the ejector 3 and then also in the sewer network 2. Between the outlet end 2a of the sewer network and the suction chamber of the ejector 3 there is a non-return valve 9 (see Figure 2) and a normally-open shut-off valve 10. The working medium of the ejector 3 and the air and sewage drawn through the sewer pipes to the ejector 3 flow at high speed through a discharge pipe 11 of the ejector directly into the interior of the container 5. Upstream of the circulation pump 6 there is a normally-open shut-off valve 12 and a grinding device 13 that grinds up solid matter occurring in the sewage. The grinding device 13 may be driven by the circulation pump 6 and it may be connected to the pump, for example, integrated so that it is on the same shaft as the pump rotor. The flow rate generated by the pump 6 is, in the embodiment being discussed, more than 100 m 3 /h. The pressure in the pipe 8, just upstream of the ejector 3, is then about 2 bar gauge. The pump 6 is capable of emptying the container 5 and driving the ejector 3 at the same time. In the emptying phase, a preferably remotely controlled emptying valve 14 is opened, whereby a proportion, for example 20% of the medium flow passing through the pump 6 flows from the pipe 8 to a pipe 15. The power of the pump 6 is high enough that, even when the ejector 3 is operating at adequate power, the medium flow pumped to the pipe 15 may rise a distance h that is about 10 to 20 metres above the level of the pump 6. In the container 5 there are two level indicators 16a and 16b, of which the lower 16a actuates an alarm, if there is too little liquid in the container and the upper 16b actuates an alarm, when the liquid level rises so high that the container 5 requires emptying. However, the sewer system illustrated is operable even if the liquid level in the container 5 rises above the level set by the upper level indicator 16b and even in the case that the discharge pipe 11 of the ejector 3 is partly or totally below the liquid level in the container 5. Normally, however, the liquid level should always be clearly below the discharge pipe 11 of the ejector 3, for instance by a distance of 1.5 to 2 times the diameter of the bore of the discharge pipe below the longitudinal axis of the discharge pipe. The distance d from the outlet end of the discharge pipe 11 to the closest wall (or other obstacle) in front of it should not be less than a certain minimum distance which is recommended to be 0.5 metre, increasing to 1.0 metre in the case of ejectors operating at the highest ejector powers contemplated. A vacuum generating means according to the invention may be advantageously used, for example, in a large passenger ship. In this case about 200 W.C. toilet bowls may be connected to one network powered by a single ejector. Several ejector arrangements according to Figure 1, each including its own circulation pump 6, can be connected to feed into the same collecting container 5. They are then conveniently all connected through one common pipe to the same sewer network 2. The volume of the collecting container 5 is usually 10 m 3 or more. It is maintained at atmospheric pressure. All the ejectors 3 connected to the same collection container 5 do not need to provide a collection-container-emptying facility if it is not necessary to increase the emptying speed by using several circulation pumps 6 simultaneously for emptying the collection container 5. The component parts of the ejector 3 are shown in greater detail in Figure 2. The check valve 9 in the suction pipe 4 of the ejector 3 has the form of a flexible rubber flap that, when the ejector is running, moves up into a position 9a in an enlargement 4a of the suction pipe 4. In the casing of this enlargement a detachable inspection cover 17 is provided, which after removal provides free access to the interior of the suction chamber of the ejector. The delivery pipe 8 for the working medium (Figure 1) of the ejector 3 is connected to a flange 18. A nozzle member 19 is held between the flange 18 and the ejector casing 22 by screw bolts 20. Hence the nozzle member 19 is easily exchangeable for a different nozzle, should one wish to change the characteristics of the ejector. The angle v between the longitudinal axis 21 of the ejector and the longitudinal axis 4b of the suction pipe 4 is about 45° in the embodiment illustrated. The cylindrical discharge pipe 11 of the ejector 3 is, attached to the ejector casing 22 by means of a flange connection 24. The discharge pipe 11 is thus easily removable and exchangeable, if for example, an exchange of the nozzle member 19 requires the use of a different discharge pipe. The discharge pipe 11 and at the same time the whole ejector 3 is connected to the collecting container 5 by means of a flange 25, which by means of a collar (not shown), can be adjustably mounted on the pipe 11, so that it may be relocated in the longitudinal direction of the pipe 11. The length L of the discharge pipe 11 is 8 to 20, preferably 10 to 15 times its inner or bore diameter D. The cross-sectional area of the free opening of the discharge pipe 11 is in the embodiment illustrated slightly more than 2.5 times the area of the smallest aperture 26 of the nozzle member 19 of the ejector 3. The invention is not limited to the embodiments disclosed, since several variations thereof are feasible, including variations which have equivalent features, within the scope of the accompanying claims.

Claims: Vacuum generating means for providing reduced pressure for sewage transport in a vacuum sewer system, which means comprises a liquid-driven ejector (3), the working medium of which is fed to the ejector (3) by a circulation pump (6) from a sewage collecting container (5), the suction side (4) of the ejector 3 being, via a check valve (9) connected to a vacuum sewer network (2), so that air and sewage delivered to the sewer network are drawn through the ejector (3) into the collecting container (5), characterised in that the bore of the discharge pipe (11) of the ejector, through which air and sewage drawn from the sewage network (2) and the working medium of the ejector provided by the circulation pump (6) are discharged into the collecting container (5), is substantially cylindrical over a length (L) that is 8 to 20, preferably 10 to 15, times the diameter (D) of the bore and in that the pipe (11), as known per se, is arranged to discharge directly into the open interior of the collecting container (5). Vacuum generating means according to claim 1, characterised in that the pressure generated in the working medium by the circulation pump (6) just upstream of the ejector (3) is at least 1.5 bar gauge, preferably at least 1.9 bar gauge. Vacuum generating means according to claim 1 or 2, characterised in that the flow of working medium fed to the ejector (3) by the circulation pump (6) is at least 90 m 3 /h, preferably 100 m 3 /h or more. Vacuum generating means according to any one preceding claim, characterised in that the free cross-sectional area of the bore of the discharge pipe (11) of the ejector is at least 2.2 times, preferably at least 2.5 times, the cross-sectional area of the smallest aperture (26) of the nozzle (19) for the working medium provided in the ejector (3). Vacuum generating means according to any one preceding claim, characterised in that the end portion (2a) of the vacuum sewer network (2) connected to the ejector (3) connects to a suction chamber (4a) in the ejector (3) at an orientation directed towards the discharge pipe (11) of the ejector (3) and inclined at an angle of 45° ± 20°, preferably at an angle of 45° ± 10°, to the longitudinal axis of the discharge pipe (11). Vacuum generating means according to any one preceding claim, characterised in that a nozzle member (19) and the discharge pipe (11) of the ejector are removably attached to other structural parts of the ejector (3) so as to be interchangeable with other parts for changing the pumping characteristics of the ejector (3). Vacuum generating means according to any one preceding claim, characterised in that the power of the circulation pump (6) is so chosen relative to the required working medium flow of the ejector (3), that the pump (6), even when the ejector (3) is operating as a vacuum generator, is capable of pumping a part of the contents of the collection container (5) to a height that is at least 10 m, preferably at least 15 m, above the level of the pump (6). Vacuum generating means according to any one preceding claim, characterised in that the clearance (d) is provided between the outlet end of the discharge pipe (11) of the ejector (3) and the closest obstruction in front thereof is at least 0.5 m, preferably at least 1.0 m. Vacuum generating means according to any one preceding claim, characterised in that upstream of the circulation pump (6) there is a grinding device (13) that grinds up sewage flowing into the circulation pump. Vacuum generating means according to any one preceding claim, characterised in that an openable inspection cover (17) is provided to facilitate access to the interior of the ejector (3).

Inventors: NURMI PEKKA, NURMI, PEKKA
Applicants: EVAC AB
Application year: 1994
Main classification: E03F1
IPC codes: E03F7, E03F3, E03F1